1. Field of the Invention
The present invention relates to an emulsion of a vinyl acetate-ethylene copolymer and an aqueous emulsion adhesive composition containing said emulsion.
2. Prior Art
It has hitherto been known that emulsions of vinyl acetate-ethylene copolymers have good adhesive properties of a wide variety of materials including not only those having good affinity with aqueous emulsion adhesives, such as wood, paper, fibers and aluminium, but also hydrophobic resins, such as polyvinylchloride resins and polystyrene resins. However, the known emulsions of vinyl acetate-ethylene copolymers having high vinyl acetate content cannot be used for binding films made of materials which are more hydrophobic and poorer in surface activities, such as ABS resins, polyester resins, two-directionally elongated polyamide resins and polyolefin resins, since they are deficient in adhesive properties thereto. The known emulsions of this type have further disadvantages that they are inferior in resistances against heat, water-proof properties and solvent-proof properties.
In order to eliminate the aforementioned disadvantages of the emulsions of binary copolymers prepared from vinyl acetate and ethylene, it has been tried to use an emulsion of ternary copolymer which has been prepared by polymerizing vinyl acetate and ethylene monomers with a third monomer of unsaturated carboxylic acid or ester thereof. However, the addition of an acid monomer is accompanied with disadvantageous phenomenon of separation of homopolymer of the added acid monomer or copolymer mainly composed of the added acid monomer in the water phase in a large quantity, leading to disrupture of emulsion or formation of coarser particles.
On the other hand, it has been proposed to prepare a thermosetting composition containing said emulsion of vinyl acetate ethylene copolymer as the main ingredient added with an initial stage condensation product of an amine resin, such as melamine-formaldehyde resin or urea-formaldehyde resin, and a hardener catalyst for the amino resin so as to improve the water-proof property and the solvent-proof property of the composition and to obtain a composition suited for use as an adhesive for electrostatic flock transfer sheet. However, such an adhesive has disadvantages that a pollution problem at the working site and a problem of residual poison are caused by formalin. Even if formalin is not used in the preparation of adhesive of this type, the known thermosetting adhesive compositions have disadvantages that they require high temperature for setting and that they cannot be used for certain kinds of materials.
An aqueous emulsion type adhesive containing a synthetic resin is used for binding the laminate layers to manufacture a composite laminated film or plate when the laminate layers are made of materials easily to be adhered with each other, for example made of paper and a metal foil or paper and a plywood. However, in general, the aqueous emulsion type adhesives are unsatisfactory in their adhesive powers when used for binding the films having hydrophobic surfaces, such as printing sheets and plastics films made of polyethylene, polypropylene, polyesters, polyvinylchloride and polystyrene, particularly two-directionally elongated polystyrene. In addition, they do not form dried membrane which are satisfactory in transparency, luster and water-proof property capable of withstanding boiling for sterilization. For this reason, adhesives containing organic solvents are commonly used for binding the hydrophobic materials of the aforementioned kind. However, in recent years, the use of organic solvents has not been favorably accepted by the public accompanying with the increasing social interest on the elimination of air pollution, bad odor and industrial pollution problems or the increasing demand for improving environmental sanitation at the working sites. Particularly, in the field of packaging or wrapping food stuffs, adverse effects of residual organic solvents on the flavor or taste of the foods or poisons caused thereby attract social concern, and development of an adhesive containing no poisonous organic solvent is earnestly desired.
The technique of modifying the emulsions of polyvinyl acetate by the addition of an epoxy resin and a hardener for epoxy resin has been made known in the art, for example by British Patent Specification No. 949,460 which discloses a method of modifying an emulsion of vinyl polymer by mixing an epoxy resin and a hardener for epoxy resin and by the specification of German Democratic Republic Patent No. 82518 which discloses a method of the preparation of an adhesive or paint composed, in combination, of a homo-emulsion of a polyvinyl acetate, an epoxy resin and an amine hardener. However, the known aqueous emulsion compositions disclosed in the preceding patent specifications do not exhibit satisfactory adhesive properties when used to bind hydrophobic plastic films made of, for example, polyethylene, polypropylene, polyesters, hard polyvinylchloride, two-directionally elongated polystyrene and two-directionally or unidirectionally elongated polyamide resins. Particularly when used for making a dry laminate from polyolefin films, the known modified emulsion type adhesives do not exert sufficient adhesive powers onto the polyolefin films which are poor in surface activities, and might rather impair the transparency, water-proof property or rigidness (or stiffness) which are important for improving commercial value or necessary in the package materials. Similar evaluations could be given to the known adhesives mainly composed of an emulsion of a vinyl acetate-ethylene copolymer, which has been considered as the most suitable main adhesive component, containing a high content of vinyl acetate groups and added with an epoxy resin and a hardener for epoxy resin as mentioned above. When an emulsion of a vinyl polymer is used as the main component and added with a modifier to prepare an adhesive composition, the main component per se must have strong adhesive power to plastic films. An adhesive somewhat improved in adhesive property to plastic films can be prepared when polyvinyl alcohol is used to form a protective colloid in the preparation of an adhesive mainly composed of an emulsion of a vinyl acetate ethylene copolymer. However, the resultant emulsion of vinyl acetate-ethylene copolymer has disadvantages that it forms a dried membrane which is poor in transparency, that the membrane becomes white when subjected to boiling for sterilization and that the adhesive power thereof is not yet satisfactory. Moreover, the binary emulsion of vinyl acetate-ethylene prepared from vinyl acetate monomers and ethylene monomers has another disadvantage that it lacks functional groups necessary for binding with the functional groups of an added modifier, such as epoxy resin, to form a three-dimensional network, resulting in formation of a laminated film which is deficient in stiffness necessary for use as a package material, so that such a laminated film might encounter difficulties at the step of filling goods therein, or suffers from blistering at the heat-seal step imparing the appearance thereof as a package material.